1) Field of the Invention
The present invention relates to an image forming apparatus that uses an electrophotographic process, such as an electrostatic copier, a laser printer or a plain paper facsimile machine, and an image forming method for the apparatus. More particularly, the present invention relates to an image forming apparatus that is equipped with a multi-beam optical system having a plurality of light emitting sources (laser diodes or the like) in a writing unit and that performs a pseudo half tone process on input image data to generate output image data and performs writing by executing light modulation of the light emitting sources of the writing unit, and an image forming method for the apparatus.
2) Description of the Related Art
FIG. 18 is a schematic structural diagram of an imaging unit (printer unit) in a conventional image forming apparatus that functions on an electrophotographic process. The imaging unit includes a photosensitive drum 101 that has a photoconductor coated on the top surface of a conductor. The photosensitive drum 101 rotates in the direction of an arrow A. The imaging unit also includes a charging roller 102, an exposure unit 103, a developing unit 104, a transfer/feeding unit 105, and a cleaning unit 106 that are arranged around the photosensitive drum 101. The imaging unit also includes a fixing unit 107 at the downstream side of the transfer/feeding unit 105.
The image forming apparatus forms an image in the following manner:
1. The charging roller 102 charges a surface of the photosensitive drum 101 to a desired potential,
2. Optical writing is done on the charged top surface of the photosensitive drum 101 with a laser beam LB from the exposure unit (optical unit) 103 to form an electrostatic latent image corresponding to the desired image,
3. The electrostatic latent image formed on the surface of the photosensitive drum 101 is developed with a toner in the developing unit 104, thereby forming a toner image,
4. The transfer/feeding unit 105 transfers the toner image on the photosensitive drum 101 onto a recording sheet 110, such as paper, which is fed in the direction of an arrow B at a given timing by a sheet feeder, such as resist rollers (not shown), and feeds the recording sheet 110 in the direction of an arrow C,
5. The cleaning unit 106 cleans toners that are not transferred onto the recording sheet 110 and remains on the surface of the photosensitive drum 101,
6. The recording sheet 110 with the toner image is transported in the direction of the arrow C by the transfer/feeding unit 105 and fed to the fixing unit 107. The fixing unit 107 heats the recording sheet 110 to thereby fixing the toner image. After the toner image is fixed on the recording sheet 110, the recording sheet 110 is discharged in the direction of an arrow D.
As the photosensitive drum 101 rotates in the direction of the arrow A, subsequent desired image is formed on the recording sheet 110 by repeating the steps 1 to 6 mentioned above.
The exposure unit 103 in the electrophotographic process is generally designed to perform light modulation of a laser diode (LD) in association with an output image. The exposure unit 103 includes an LD that irradiates a laser beam onto the photosensitive drum 101 via a collimate lens, an aperture, a cylindrical lens, a polygon mirror, an f-θ lens, etc. (none of which are not shown).
The polygon mirror is a rotatable mirror with multiple surfaces. As the polygon mirror rotates, the laser beam LB scans on the top surface of the photosensitive drum 101 (main scanning).
As the photosensitive drum 101 rotates in a direction orthogonal to the scan direction of the laser beam LB by a photoconductor driving unit (sub scanning), it is possible to expose the top surface of the photosensitive drum 101 with the laser beam LB to two-dimensionally form an electrostatic latent image corresponding to the desired image on the top surface of the photosensitive drum 101.
FIG. 19 is an exemplary configuration of the conventional image forming apparatus.
The conventional image forming apparatus includes an image input unit 111 which is a scanner or the like. The scanner may be the one that is connectable to computers, or that is arranged in digital copying machines or image data reading units. The image input unit 111 sends image data, which may be read from an original or may be read from a recording medium, as input image data PDi to an image processing unit 112.
The image processing unit 112 performs various kinds of image processing, such as MTF filtering, gradation correction (γ conversion), and a pseudo half tone process, on the input image data in order in an MTF filtering unit 113, a gradation correcting (γ conversion) unit 114, and a pseudo half tone processing unit 115, and sends output image data PDo as a processing result to a video signal processing unit 117.
The video signal processing unit 117 converts the output image data PDo to an image signal PS, sends the image signal PS to the exposure unit 103 shown in FIG. 18, and drives the LD at a given timing. In an image forming apparatus equipped with a plurality of LDs, the video signal processing unit 117 distributes the image signal for the LDs to be used.
The individual units are connected to a Central Processing Unit (CPU) 121, a read only memory (ROM, program memory) 122, and a random access memory (RAM, data memory) 123, that constitute a microcomputer, and an operation unit 124 having operation keys and a display via a system bus 120, and are controlled by the CPU 121.
The image forming apparatus described above is a monochromatic electrophotographic image forming apparatus. As a full-color electrophotographic image forming apparatus, there is known a tandem type electrophotographic apparatus that has four sets of electrophotographic process units (equivalent to the individual units shown in FIG. 18) respectively corresponding to the individual colors of cyan (C), magenta (M), yellow (Y), and black (K).
In a tandem type full-color image forming apparatus, toner images of the colors C, M, Y, and K are transferred onto a belt-like intermediate transfer in an overlaid manner, then the toner images of the four colors are transferred onto a recording sheet, such as paper, at a time. The toner images on the recording sheet are heated and pressed by a fixing unit to be fixed on the recording sheet. The recording sheet is then ejected out of the apparatus.
There is know a direct transfer type full-color image forming apparatus that does not have an intermediate transfer unit and overlays toner images of individual colors of C, M, Y, and K on a recording sheet in order.
There is known a revolver type full-color image forming apparatus in which developers of the individual colors of C, M, Y, and K are rotatably supported with respect to a single photosensitive drum in such a way that the developers face the photosensitive drum in order.
Generally the image forming apparatuses are equipped with a multi-beam optical system that has a plurality of light emitting sources (laser diodes or the like) in a writing unit equivalent to the exposure unit 103. If higher resolution is required or if faster printing speed required, and if only one light emitting source is used, then the polygon mirror is required to be rotated more times. This causes an increase in the noise generated from the polygon mirror, an insufficient strength of the rotary shaft thereof, an increase in heat generated by the rotary shaft, and an enlargement of a driving power source.
Further, the frequency of a pixel clock should be increased at the same time, requiring the fabrication of an electronic circuit adapted for high-speed switching of the laser diode. When the drive frequency of an electronic circuit exceeds 50 Megahertz, however, it becomes difficult to stably operate the electronic circuit.
As a solution to those problems, there is known a multi-beam system having a plurality of light emitting sources. The multi-beam system simultaneously scans with laser beams emitted from the light emitting sources using the polygon mirror, thereby simultaneously forming plural lines of electrostatic latent images on a photoconductor, as disclosed in, for example, Japanese Patent Application Laid-open No. H7-242019.
Image data to be input to the image forming apparatus is multi-value data of 8 to 12 bits per pixel for a gradation image of a picture or the like. In an image forming apparatus (including an electrophotographic type) that forms an image on a sheet of paper (so-called hard copy), the number of gradations that can be expressed per pixel is substantially very small.
To overcome the problem, an image forming apparatus as a hard copying machine displays a half-tone image in a pseudo manner by improving the resolution to 600 dots per inch (dpi) or to 1200 dpi and modulating the image density with a plurality of pixels in terms of an area. The image processing that is performed in the process of converting the input image data to data of a pseudo half tone image is a pseudo half tone process.
Input image data can be classified into the following three types of images.
(1) Character/line image
(2) Picture image
(3) Graphics image
The character/line image of the type (1) has a characteristic that the shape reproducibility of a character/line is important whereas the color reproducibility and the gradation reproducibility are less significant. The picture image of the type (2) and the graphics image of the type (3) have the opposite characteristic such that the color reproducibility and the gradation reproducibility are more significant than the shape reproducibility.
Under such a circumstance, as a conventional technology, an electrophotographic apparatus described in, for example, Japanese Patent Application Laid-open No. H9-282471 employs a method of changing the number of lines in a pseudo half tone process in such a way that a screen process with 400 lines is performed for a character/line image and a contour and a screen process with 200 lines is performed for other images, such as a picture image.
The resolution of an electrophotographic image forming apparatus has a tendency of becoming greater. At present, the standard resolution is 600 dpi and there are many apparatuses that achieve the resolution of 1200 dpi.
In most of the apparatuses that achieve the resolution of 1200 dpi, the beam size of a laser beam to expose a photoconductor (the size of a spot that forms an area indicating 1/e2 of the peak amount of light) lies in a range of 50 to 80 micrometers. With the resolution of 1200 dpi, the length per pixel is 21.2 micrometers, which is significantly smaller than the beam size.
The beam size of the laser beam is determined by the wavelength of a laser, the focal distance of the optical system, and the aperture size, so that reducing the beam size alone causes a problem of enlarging the apparatus. This prevents the beam size from being made smaller actively.
Setting the resolution to 1200 dpi is advantageous in that jaggies (an oblique line or a periphery of a character having a jagged contour) in a character/line image can be eliminated and oblique lines or characters having a smooth contour can be reproduced. As a result, the differences between individual fonts can be discriminated, thus ensuring such printing as to make the differences between individual fonts discriminable even in a hard-copy image like an image formed on a sheet of paper.
However, the experiments conducted by the present inventor showed that increasing the resolution as large as 1200 dpi resulted in significant degrading of the gradation including a reduction in the reproducibility of highlighting. This has made it clear that a picture image or a graphics image for which the gradation reproducibility or the color reproducibility are considered significant suffer a reduction in image quality. This seems to have originated from increasing the resolution alone without reducing the beam size.
That is, for images undergone a pseudo half tone process with the same number of lines, an image with a higher resolution has a larger number of pixels to be written. However, the size of an area on a photoconductor to be actually exposed with a laser beam in association with one pixel is determined by the beam size of the laser beam, so that increasing the resolution increases the area of a region on the photoconductor to be exposed becomes larger unless the beam size becomes smaller. This seems to degrade the gradation including a reduction in the reproducibility of a highlighted region.
The description given above is explained in reference to FIG. 20. FIG. 20 depicts, as an example, an image with 200 lines per inch (lpi) as the number of lines and an area ratio of 11% (= 1/9). With the resolution of 600 dpi, image data has ON data included at only one pixel in a region consisting of a total of nine pixels, three pixels in the main scanning direction and the sub scanning direction. While FIG. 20 depicts only a region of 3×3 pixels, an image having such a pattern repeated cyclically is formed. This image data is shown on the upper left side in FIG. 20.
When an image with 200 lpi and an area ratio of 11% is formed with a resolution of 1200 dpi, image data has ON data included at four pixels in a region consisting of 6×6 pixels or a total of 36 pixels, as shown on the upper right side in FIG. 20.
Here is an instance that exposure is done with a laser beam having a beam size of 60 (main scanning direction)×60 (sub scanning direction) micrometers with respect to such image data with both resolutions of 600 dpi and 1200 dpi. The reason why it is not easy to reduce the beam size according to the resolution has already been given above.
The lower side in FIG. 20 depicts the photoconductor being exposed corresponding to the image data. In practice, the photoconductor is exposed by turning on the laser diode while scanning the photoconductor in the main scanning direction with the laser beam, so that the exposure region moves in the main scanning direction. Accordingly, the exposure region becomes elongated in the main scanning direction. However, as the fact is not essential in the description below, it will not be explained.
While the beam size is the same for 600 dpi and 1200 dpi, the amount of light corresponding to one pixel differs between 600 dpi and 1200 dpi. Ideally, writing is done for 600 dpi with the amount of light four times the amount of light for 1200 dpi, but writing is done with the adequate amount of light for each resolution.
The lower left side in FIG. 20 depicts the exposed state on the photoconductor for 600 dpi, and the lower right side depicts the exposed state on the photoconductor for 1200 dpi. When writing is done with the same beam size for 600 dpi and 1200 dpi, the exposure energy is dispersed for the pattern written with the higher resolution of 1200 dpi (a wider region on the photoconductor is exposed). When the same image pattern (the pattern with 200 lpi and an area ratio of 11%) is formed with different resolutions, the exposure energy is dispersed for the pattern with a higher resolution.
As a result, an electrostatic latent image to be formed by the writing process is formed shallow over a wide region. When an electrostatic latent image is formed shallow, a phenomenon, such as deteriorated highlight reproducibility, occurs at a highlighted portion, and deformation becomes quicker at a dark portion due to the same mechanism, thereby lowering the gradation reproducibility.
For an oblique line or the like, so-called jaggies can be reduced by setting the resolution to 1200 dpi. FIG. 21 depicts image data (upper side) corresponding to oblique lines with the same line width and an inclination angle of 45 degrees and exposure regions (lower side) corresponding to the image data.
With the resolution set to 1200 dpi, image data itself can be generated with reduced jaggies of an oblique line (the upper right side in the diagram).
An exposure region on the photoconductor when writing is done for the image data becomes as shown on the lower right side in FIG. 21. While the exposure energy is dispersed and spread in the exposure region as per the previous case of a dot image, it is apparent that the jaggies at the edge portions are reduced. For a character/line image, therefore, increasing the resolution to 1200 dpi from 600 dpi can ensure expression of an image with reduced jaggies.
When the resolution is increased without reducing the beam size, the photoconductor is exposed with the exposure energy dispersed for the reason given above. This may lower reproducibilities including the highlight reproducibility at a highlighted portion and the gradation reproducibility, and may lower the gradation for a picture image or a graphics image, degrading the image quality.
Such a reduction in gradation reproducibility means the generation of a dense region indicating a sharp density change in the relationship between input image data and an image density (so-called γ characteristic). The appearance of such a region indicating a sharp density change means the appearance of discontinuous gradation in an output image, which is a major factor of degrading the image quality.
In color image formation of outputting an image through color correction or gradation correction, a gradation loss occurs in the region indicating a sharp density change at the time of correcting the gradation. This means the degradation of the quality of the image of a pseudo contour or the like.